Roller Hemming Head Having a Replaceable Roller-Carrying Unit

ABSTRACT

A roller hemming head having a quick-connection device which rigidly and removably connects a roller-carrying unit to the head lower portion. The quick-connection device includes spring means and an actuating member selectively engaged by an engagement member provided at a fixed station for quick replacement of the roller-carrying unit without cables and/or supply hoses for controlling the quick-connection device.

FIELD OF THE INVENTION

The present invention relates to roller hemming heads.

BACKGROUND OF THE INVENTION

A roller hemming head of the above indicated type is disclosed for example in WO 2012/160512 of the same Applicant. In this known solution, the head has a lower portion comprising an outer cylindrical body slidably mounted on an inner cylindrical body forming part of the upper head portion. The lower head portion directly and freely rotatably supports a pair of hemming rollers projecting from opposite sides of the head and adapted to be used selectively during a hemming operation. The hemming rollers have a common axis orthogonal to the common axis of the two outer and inner cylindrical bodies of the head.

The exemplary embodiment shown in the above cited document WO 2012/160512 provides for the hemming head to operate both according to a “push” mode, i.e. with one of the two hemming rollers being pushed with its lower side (with reference to a vertical orientation of the head) against a piece to be hemmed, and according to a “pull” mode, i.e. with the head being subjected to a pulling action, so that the upper side of one of the hemming rollers (again with reference to a vertical orientation of the head) is engaged against the piece to be hemmed.

The known hemming head comprises an upper helical spring and a lower helical spring arranged aligned along the common axis of the outer and inner cylindrical bodies of the head.

The ends of the upper and lower springs which are more remote from each other are supported against end elements of the inner cylindrical body, whereas the ends of the two springs which are closer to each other rest on opposite faces of an intermediate support element which is rigidly connected to the outer cylindrical body. In this manner, during the operation according to a “push” mode, it is the upper spring which opposes the force tending to compress the head, whereas in the “pull” mode, it is the lower spring which opposes the pulling force applied to the head.

In the above mentioned known solution, a quick-release device is also provided for uncoupling a single hemming roller from the head lower portion. The release of the roller can be activated by any operating member located outside of the head, which is engaged against a release member provided adjacent to each hemming roller.

Solutions are also known of hemming heads which include a quick-connection device at their upper end, through which the entire hemming head carried by the robot can be quickly replaced. These solutions are however uselessly complicated and expensive, in cases where only the hemming rollers would need to be replaced. Moreover, the quick-connection devices used in this case require providing the robot with electric cables and hoses for supplying fluid under pressure, which are necessary for controlling the operation of the quick-connection device. Apart from their intrinsic cost, these cables and hoses have the drawback to be subjected to continuous deformations during the operative life of the robot, so that they must be periodically replaced due to wear thereof, thus giving rise to additional costs and productivity losses caused by the maintenance stops. Wear of cables and hoses is particularly high in the case of a hemming head, since this head is often used for performing hemming operations along the entire periphery of a component, which implies that the head rotates by 360° on itself, which therefore subjects cables and supply hoses of the quick-connection device to very high torsional deformations.

SUMMARY OF THE INVENTION

The main object of the present invention is that of providing a roller hemming head which enables the hemming rollers to be replaced and yet has a simple and inexpensive structure, which does not require the provision of additional electric cables and/or supply hoses on the robot carrying the hemming head.

A further object is that of providing a hemming head which achieves the above mentioned purpose through a new and advantageous head architecture, while not involving substantial increases in cost with respect to the conventional hemming heads.

A further preferred object is that of providing a hemming head which is more efficient and less complicated in use with respect to the known solutions.

In view of achieving these objects, the invention provides a roller hemming head comprising:

an upper head portion, having a flange for connection to a robot wrist or other movable supporting equipment, and

a lower head portion, carrying one or more hemming rollers, characterized in that said head further includes:

a roller-carrying unit carrying the hemming rollers and adapted to be separated as a whole from said head lower portion, and

a quick-connection device which rigidly and removably connects said roller-carrying unit to the head lower portion,

said quick-connection device comprising:

spring means tending to hold said device in a connected condition,

an actuating member movable against the action of said spring means for releasing the connection, and adapted to be operated by an engagement member, said engagement member being provided at a fixed station adjacent to said robot or adjacent to said other movable equipment supporting the head and being adapted to be received within a cooperating seat of the head,

in such a way that said quick-connection device does not require the provision of cables and/or supply hoses on said robot or on said other supporting movable equipment.

Therefore, in the present invention the hemming rollers of the head are all carried by a single roller-carrying unit which can be separated as a whole from the lower head portion and thus quickly replaced. This represents an important difference with respect to the solution known from WO 2012/0160512, in which the possibility of quick change of each single roller was provided, and also with respect to conventional solutions in which quick change of the entire hemming head is provided due to the use of a quick-connection device interposed between the hemming head and the robot wrist.

Furthermore, in the hemming head according to the invention the device for quick change of the roller-carrying unit does not require the provision of electric cables and/or additional supply hoses on the robot, which simplifies and lowers the cost of the robot structure. The elimination of cables and the hoses on the robot is two-fold advantageous, since these cables and hoses are subjected to continuous deformations during the operative life of the robot, with the result that they must be periodically replaced, with additional costs and productivity losses due to maintenance stops.

The present invention also provides a hemming fixed station, comprising a robot or other supporting movable equipment, and a hemming head having the above indicated features, carried by said robot or by said other movable supporting equipment.

According to a further feature, the hemming head according to the invention further comprises a fixed station adjacent to said robot or to said movable equipment supporting the head, said fixed station including an engagement member which can be engaged within a cooperating seat of the head when said robot or said movable supporting equipment brings the head to a position adjacent to said fixed station, in order to activate the actuating member for release of the connection between the lower portion of the hemming head and the roller-carrying unit.

Preferably, at said fixed station there is provided also a movable magazine for roller-carrying units, comprising a plurality of structures for receiving the roller-carrying units, which can be moved along an open or closed path, so that an empty receiving structure can be provided at a position for receiving a roller-carrying unit when the latter is uncoupled from the hemming head, and a receiving structure occupied by a new roller-carrying unit can therefore be brought to said position for coupling the new roller-carrying unit with the hemming head.

In the hemming head according to the invention, the quick-connection device connecting the roller-carrying unit to the head lower portion can be made according to any conventional technique.

However, in one exemplary embodiment, this device is made according to the teachings of document EP 1 810 777 A1, assigned to Pascal Engineering Corporation. The quick-connection device disclosed in this document is for connection of piece-carrying pallets onto pallet-carrying structures provided along a pallet conveyor device. However, the Applicant has found that this connection device can be advantageously used also for the purposes of the present invention.

In the case of this known device, the actuating member which causes release of the connection is operated by hydraulic means. Therefore, in case the present invention makes use of this known device, the above mentioned engagement member provided at the fixed station is adapted to establish a hydraulic communication between a hydraulic chamber provided within the connection device and a source of pressurized hydraulic fluid. However, operation of the actuating member for release of the connection could be obtained in any other way, such as by means of pressurized air or also by mechanical means. In this latter case, the engagement member which is received within the cooperating seat on the hemming head comprises for example a pushing member which engages a corresponding actuating member provided within the seat of the head in order to cause release of the connection. Whatever is the solution which is adopted, when the engagement member is retracted from the corresponding seat in the head, the spring means provided in the connection device automatically cause return of this device to a condition of rigid connection between the new roller-carrying unit and the lower portion of the hemming head.

Moreover, the head according to the invention preferably has the further features which are indicated in the annexed dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be now described with reference to the annexed drawings, given purely by way of non-limiting example, in which:

FIG. 1 is a diagrammatic front view of a hemming head according to the prior art, carried on the terminal portion of a multi-axis industrial robot;

FIG. 2 is a view in partial longitudinal cross-section and partial cut-away and at an enlarged scale of the hemming head of FIG. 1;

FIG. 3 is a perspective view which shows a further known solution, in which a device is provided for quick replacement of the hemming head carried by a robot;

FIGS. 3A, 3B are diagrammatic front views which show a hemming head with the conventional head change device which is shown in FIG. 3, compared with a head which is not provided with this device;

FIG. 4 is a perspective view of one embodiment of the hemming head according to the invention;

FIG. 5 is a diagrammatic longitudinal cross-sectional view of the hemming head of FIG. 4,

FIG. 5A shows a detail of FIG. 5 at an enlarged scale;

FIG. 6 is a diagrammatic view in cross-section of a quick-connection device known per se, which can be used in the hemming head of the invention; and

FIG. 7 shows an exemplary embodiment of a hemming fixed station according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows one example of hemming head according to the prior art known from WO 2012/160512. In this figure, reference numeral 10 generally designates a hemming head carried by a multi axis industrial robot R, of which only the terminal part is visible, having a wrist 14 able to move and orient the head 10 in a three-dimensional space. In an exemplary application, robot R is electronically connected to a controller (not shown) which is programmed for moving and orienting the hemming head 10 along a predetermined path for performing a hemming operation, according to what will be described more in detail in the following with reference to FIG. 7 of the annexed drawings.

The head 10 according to the prior art includes an upper portion 20 rigidly connected to a universal connecting flange 22, of circular shape, for connection to the robot wrist. The flange has a plurality of holes for engagement of connecting bolts (not shown) for connection to the robot wrist, according to a technique which is conventional per se. The hemming head 10 further comprises a lower portion 24 which freely rotatably supports two hemming rollers 26, 27 around a common axis 28 which is orthogonal with respect to the longitudinal axis 100 of the hemming head 10.

As shown in detail, in FIG. 2, in the head 10 according to the prior art the lower head portion 24 comprises an outer cylindrical body 240 slidably mounted on an inner cylindrical body 200 forming part of the upper portion 20 of the head, which is connected to the connection flange 22. The head 10 according to the prior art further comprises two helical springs 30A, 30B aligned along the head axis 100. The ends of springs 30A, 30B which are more remote from each other rest against end elements of the inner cylindrical body 20, whereas the ends of springs 30A, 30B which are closer to each other rest against opposite faces of an intermediate element 32 which is rigidly connected to the outer cylindrical body 240, through diametrically opposite slots formed in the inner cylindrical body 200.

The hemming head 10 shown in FIGS. 1, 2 can be used both according to a “push” mode and according to a “pull” mode. In the “push” mode, the robot R pushes the hemming head 10 against a piece so as to use one of the two hemming rollers 26, 27 for performing the hemming operation with the portion of the roller which faces downwardly (with reference to FIGS. 1, 2). In the “pull” mode the robot R pulls the hemming head 10 towards the flange 22, i.e. upwardly with reference to FIGS. 1, 2, so as to use the upwardly facing side (with reference to the figures) of one of the hemming rollers 26, 27 for performing the hemming operation. In the “push” mode, the inner cylindrical body 200 which is rigidly connected to the connection flange 22 tends to move downwardly (with reference to FIGS. 1, 2) with respect to the outer cylindrical body 240 carrying the hemming rollers, thus causing a compression of the upper helical spring 30A. In the “pull” mode, the inner cylindrical body 200 tends to move upwardly (again with reference to the figures) with respect to the outer cylindrical body 240, causing a compression of the lower helical spring 30B.

According to what is indicated in document WO 2012/160512, each of the hemming rollers 26, 27 can be quickly replaced by means of a quick-connection device which can be operated by a robot or any other automatic or manual mechanism adapted to engage a release device provided on the head. However there is nothing in this document which anticipates or suggests the idea which is at the basis of the present invention, i.e. the provision of a unit carrying the hemming rollers which can be uncoupled, as a whole, with respect to the lower portion of the hemming head.

FIG. 3 of the annexed drawings shows a further known solution used by the same Applicant, in which a hemming head similar to that of FIGS. 1, 2 can be quickly uncoupled from the robot by which it is carried with the use of a quick-connection device 34 interposed between the connection flange 22 located at the upper end of the hemming head 10, and the robot wrist (not shown in FIG. 3). In this known solution, the quick-connection device 34 is of a type which requires the provision on the robot of electric cables 36 and hoses 37 for supplying pressurized air, which are necessary for controlling the operation of the quick-connection device 34.

Apart from the intrinsic cost, these cables and hoses have the drawback of being subjected to continuous deformations during the operative life of the robot, with the result that they must be periodically replaced because of wear thereof, thus giving rise to additional costs and productivity losses caused by maintenance stops. Wear of cables and hoses is particularly high in the case of a hemming head, since the head is often used for performing hemming operations along the entire periphery of a component, which implies that the head performs a 360° rotation on itself, subjecting cables and supply hoses of the quick-connection device to very high torsional deformations.

FIG. 3A of the annexed drawings also shows a conventional solution of the type of that of FIG. 3, in which the hemming head 10 is provided with the quick-connection device 34 and therefore requires the provision of cables and hoses for supplying the device 34. The comparison with FIG. 3B, which shows the head without device 34, makes the disadvantage clear, also in terms of a greater bulk deriving from the large size of device 34. Moreover, the cables and supply hoses of device 34 must be arranged inside the robot, so that they require the provision of corresponding supports on the robot, which generates further bulk and complications. Moreover, the provision of cables outside the robot is a possible source of problems due to interference with foreign bodies which may be located at the work fixed station. Starting from this state of the art, the invention has the object of finding a solution which overcomes the above indicated drawbacks.

FIGS. 4-6 of the annexed drawings show an exemplary embodiment of the hemming head 10A according to the present invention. In these figures, the parts in common to those of FIGS. 1, 2 have been designated by the same reference numerals.

As indicated in the foregoing, a main feature of the hemming head according to the present invention is that in this head the hemming rollers 26, 27 are freely rotatably supported by a roller-carrying unit 40 which can be separated as a whole from the lower portion 24 of the hemming head 10. To this end, between the roller-carrying unit 40 and the outer cylindrical body 240 forming part of the lower portion 24 of the head there is interposed a quick-connection device 50 of any known type, which connects a circular flange 42 provided on the roller-carrying unit 40 to a circular flange 242 secured to the lower end of the outer cylindrical body 240. In the case of the illustrated example, the connection device 50 is provided according to the teachings of document EP 1 810 777 A1 in the name of Pascal Engineering Corporation. The above mentioned document shows this quick-connection device with reference to connection of a piece-carrying pallet to pallet-carrying structures of a pallet conveyor. However, the applicant has found that this device can be advantageously used also in the application illustrated herein.

In view of that the device 50 shown in the drawings is known per se, it is described herein only with reference to its general features.

As shown in FIG. 6, device 50 comprises a first coupling member 51 which is rigidly connected to the roller-carrying unit 40 and a second coupling member 52 constituted by a piston movable inside member 51. Inside device 52 there is provided a hydraulic chamber 53 which can be fed with fluid under pressure for moving piston 52 downwardly (with reference to FIG. 6) against the action of a group of Belleville springs 54. Springs 54 are interposed between an end head 52 a connected to piston 52 by a stem 52 b, and the body of the coupling member 51. Between the coupling member 51 and piston 52 there are interposed ball elements 55. The Belleville springs 54 normally hold piston 52 in a raised position (with reference to FIG. 6) in which the ball elements 55 axially hold the coupling member 51 with respect to piston 52. When fluid under pressure is fed to chamber 53, piston 52 moves downwardly (with reference to FIG. 6) against the action of the Belleville springs 54, reaching a position in which the ball elements 55 enable axial uncoupling of member 51. In the coupled condition, the device comprises also further coupling elements (not shown in FIG. 6) for locking the two coupling members with respect to relative rotations. As indicated above, a detailed description of the device 50 is anyhow provided in document EP 1 810 777 A1, which forms part of the state of the art.

The supply of fluid under pressure to chamber 53 of connection device 50 is performed at a fixed station for change of the roller-carrying unit, shown by way of example in FIG. 7 and designated therein by reference S. At station S there is provided an engagement member 60 which is to be received within a cooperating seat (not shown in the drawings) of the connection device 50 for providing a hydraulic communication between chamber 53 of the connection device 50 and an outer source of fluid under pressure.

The connection device illustrated herein is however provided purely by way of non-limiting example, it would be possible to use a similar device in which operation of the piston element 52, which acts as actuating member for release of the device, is obtained for example by supply of air under pressure, or by a mechanical action. In this latter case, the engagement member 60 is a pushing member adapted to be received within the cooperating seat of the connection device and to operate the movable member of the connection device 50 connected in any known way to the release actuating member of the device.

It is also possible to use a quick-connection device of any conventional type, also different from that described above purely by way of example.

FIG. 7 of the annexed drawings shows, purely by way of example, the application of the invention to a hemming fixed station provided in a plant for production of motor-vehicle doors, for performing a hemming operation on two sheet-metal panels which form the structure of the door, along the periphery of the door.

According to a technique known per se, the door structure D is placed in a horizontal position above a cradle C carried by a basement B. The door structure D is pressed above cradle C by a pressing member P carried at the lower end of a column P1 which is vertically movable. The hemming operation is carried out by means of a hemming head 10A according to the invention. The head is mounted on the wrist 14 of a multi-axis articulated robot R of any known type. The robot comprises a base structure R and a plurality of robot elements mutually articulated to each other, which connect a base structure R1 to the robot wrist 14. As already indicated in the foregoing, the operation of the robot is controlled, in a way known per se, by a controller of any known type programmed for moving the hemming head 10 according to a predetermined path in a predetermined way. During the hemming operation, one or the other of the two hemming rollers 26, 27 is used, depending upon the required operation.

When it becomes necessary to replace the roller-carrying unit 40, the robot carries the hemming head 10A to the fixed station S, where there is provided a movable magazine M for structures 70 adapted to receive and support roller-carrying units 40. In the case of the example diagrammatically shown in FIG. 7, structures 70 are movable along a predetermined path by means of a conveyor 80 of any known type. In the preferred embodiment, conveyor 80 has an arrangement in the form of a horizontal carousel (only a portion of which is visible in FIG. 7) along which structures 70 are movable, in one direction or in the other. In this manner, when it is necessary to replace the roller-carrying unit 40 carried by the hemming head 10A which is mounted on robot R, at station S there is provided an empty structure 70, which is for receiving the worn roller-carrying unit 10A. Once robot R has brought the hemming head 10A above this empty structure 70, the engagement member 60 is automatically extended for engaging a cooperating seat on the quick-connection device of the hemming head 10A. In this manner, release of the connection is activated and the roller-carrying unit 40 is deposited on the correspondent structure 70. Conveyor 80 is then moved for bringing a structure 70 carrying a new roller-carrying unit 40 to station S, so that the new unit can be engaged rapidly by the hemming head 10A and then connected rigidly to the latter by disengagement of the engagement member 60, so as to bring the quick-connection device to the rigid connection condition, due to the action of springs 54. Once this operation has been performed, the robot can bring the hemming head 10A back to a piece to be hemmed, for performing a new hemming cycle.

With reference to FIGS. 5, 5A, the preferred embodiment of the hemming head 10A according to the invention has further advantageous differences with respect to the known hemming head shown in FIGS. 1, 2, which will be discussed in the following. However, it is well understood that the present invention includes providing a roller-carrying unit 40 separable, through a connection device 50, from the lower portion of the hemming head in a hemming head of any known type, including also the head of FIGS. 1, 2.

A first additional difference of the preferred embodiment with respect to the head of FIGS. 1, 2 lies in that the two springs 30A, 30B are guided by a spring-guiding stem 31 arranged axially through the two springs and through the intermediate supporting element 32.

Moreover, in the preferred embodiment the load applied to the hemming head during the hemming operation is monitored both during an operation according to a “pull” mode and in an operation according to a “push” mode, by means of two respective force sensors (load cells) 33 which are interposed between the ends of springs 30A, 30B which are more remote from each other, and the respective end elements of the inner cylindrical body 200.

The lower end of a spring-guiding stem 31 is connected to a disk 39 (FIG. 5A) on which the lower end of the lower spring 30B rests, by means of a bayonet coupling, including a transverse pin 35 carried by stem 31 and a shaped slot 37 formed in a cylindrical skirt projecting from disk 39 and engaged by one end of the transverse pin 35.

The shape of slot 37 is such that the mounting operation is carried out in the conventional manner used for bayonet couplings, i.e. by a first axial movement of disk 39, which causes a compression of lower spring 30B, followed by a rotation of disk 39 and subsequent axial release, under the action of the lower spring 30B, towards a final locked position. The locking of the coupling is therefore ensured by the lower spring 30B itself. Due to this arrangement, the mounting of the unit can be carried out in a simple and quick manner.

According to a further important difference, the load of the two springs 30A, 30B, can be adjusted by acting, at each of the two ends of the springs which are more remote from each other, on a screw 220 (FIG. 5A) which engages an internally threaded bushing 230 which is prevented from rotation with respect to the inner cylindrical body 200 by means of a key 250 and which further engages a threaded portion 260 projecting from the body of the respective sensor 33. A rotation of screw 220 modifies the axial position of the threaded bushing 230 with respect to the inner cylindrical body 200 of the head. The body of sensor 37 has a threaded portion 260 also on the side facing towards disk 39, which is received within a threaded axial hole of disk 39.

During the registering operation of the load of springs 30A, 30B, the axial relative position of the outer cylindrical body 240 and the inner cylindrical body 200 is locked by inserting a locking pin through aligned holes (not shown in the drawings) of these cylindrical bodies.

The above described system for adjustment of the load of springs 30A, 30B represents a substantial step forward with respect to what has been provided today in systems of the type shown in WO 2012/160512. In these known systems, in order to control the load of a spring it is necessary to measure the length of the spring in an undeformed condition, to measure the actual length of the space in which the spring is mounted, and then to insert a number of shims or washers which ensure the desired compression. Also in these known systems, if the spring is broken before it can be replaced it is necessary to measure a new spring, to check the measure of the length of the mounting space and to provide the required shims for reproducing the same value of load.

With the new above described adjustment system, it is possible to finally adjust the load value, by reproducing exactly the same load value even upon changing the spring (the tolerance on the spring length being large) since by screw 220 it is possible to recover the clearances and to reproduce the desired load. In this manner, it is possible to reproduce identical adjustments between different hemming heads exactly, independently from the manufacturing tolerance of the systems. Furthermore, with sensor 33 it is possible to evaluate the integrity of the spring during the use of the head.

In the case of the present invention, the above described mounting and adjustment operations can be performed easily in a condition in which the roller-carrying unit 40 is removed from the head.

Naturally, while the principle of the invention remains the same, the details of construction and the embodiments may widely vary with respect to what has been described and illustrated purely by way of example, without departing from the scope of the present invention.

In particular, as already indicated in the foregoing, the basic feature of the invention, lying in the provision of a separable roller-carrying unit 40 can be adopted in a roller hemming head of any type.

Also the arrangement of the fixed station S for change of the roller-carrying unit can be different from that which has been disclosed herein purely by way of example. 

1. Roller hemming head, comprising: an upper head portion, having a flange for connection to a wrist of a robot or other supporting movable equipment and a lower head portion, carrying one or more hemming rollers, characterized in that said head further includes: a roller-carrying unit carrying the hemming rollers and adapted to be separated as a whole from the lower portion of the hemming head; a quick-connection device which rigidly and removably connects said roller-carrying unit to said head lower portion; and said quick-connection device comprising: spring means tending to hold said quick-connection device in a connection operative condition; an actuating member movable against the action of said spring means for releasing the connection, and adapted to be operated by an engagement member, said engagement member being provided at a fixed station adjacent to said robot, or said other movable equipment for supporting the head, and being adapted to be received within a cooperating seat of the head; and in such a way that said quick-connection device does not require the provision of cables or supply hoses on said robot or said supporting movable equipment.
 2. Roller hemming head according to claim 1, characterized in that said lower head portion comprises an outer cylindrical body slidably mounted on an inner cylindrical body forming part of said upper head portion and connected to said connecting flange.
 3. Roller hemming head according to claim 2, characterized in that said hemming head comprises an upper helical spring and a lower helical spring which are arranged coaxially aligned inside said inner cylindrical body, the ends of said upper and lower springs which are more remote from each other being applied against end elements of said inner cylindrical body, the ends of said upper and lower springs which are adjacent to each other being applied against opposite faces of an intermediate support element which is rigidly connected to said outer cylindrical body through diametrically opposite elements which are rigidly connected to said outer cylindrical body and are arranged through longitudinal slots of said inner cylindrical body said upper and lower springs being provided with a spring-guiding stem arranged axially through said springs and through said intermediate support element.
 4. Roller hemming head according to claim 3, characterized in that the ends of said upper and lower springs which are more remote from each other are applied against respective end elements of the inner cylindrical body with the interposition of two force sensors.
 5. Roller hemming head according to claim 3, characterized in that it is provided with first adjusting screw means for adjusting the load of said lower spring and second adjusting screw means for adjusting the load of said upper spring.
 6. Roller hemming head according to claim 5, characterized in that each of said first and second adjusting screw means comprises a screw rotatably mounted at an axially fixed position to a respective end of said inner cylindrical body, and an internally threaded bush, which is prevented from rotation with respect to said respective end element and is engaged by said screw said bush being further engaged on a threaded portion of a respective one of said force sensors which is associated with a respective one of said springs.
 7. Roller hemming head according to claim 1, characterized in that the lower spring has its lower end which is supported against a support disk provided with a cylindrical skirt having a shaped slot cooperating with a diametrical pin projecting from said spring-guiding stem so as to provide a bayonet coupling of said support disk on the lower end of said spring-guiding stem.
 8. Hemming fixed station, comprising a robot or other supporting movable equipment, and a hemming head according to anyone of the previous claims, carried by said robot or said supporting movable equipment.
 9. Hemming fixed station according to claim 8, characterized in that it comprises a fixed station adjacent to said robot or said movable equipment for supporting the head, said fixed station including an engagement member which is adapted to be engaged within a cooperating seat of the head, after that said robot or said supporting movable equipment has brought the head to a position adjacent to said fixed station, in order to operate the actuating member for releasing the connection.
 10. Hemming fixed station according to claim 9, characterized in that at said fixed station there is provided a movable magazine for roller-carrying units comprising a plurality of structures for receiving roller-carrying units, which are movable along an open or closed path, so that an empty receiving structure can be provided at a position for receiving the roller-carrying unit when the latter is uncoupled from the hemming head, whereas another receiving unit which is occupied by another roller-carrying unit can then be brought to said position for coupling said other roller-carrying unit with the hemming head. 